While being machined, workpieces are frequently retained in a holding fixture by resting the workpiece on two or more spaced apart pads on which the workpiece is clamped. The work pads on the holding fixture are usually small in area in order to facilitate cleaning of their operative faces and also to minimize the contact area of the locating surface on the workpiece. This reduces to a minimum the distortion resulting from possibly non-planar locating surfaces on the workpiece. However, with such an arrangement relatively large portions of the workpiece are unsupported and subject to deflection by the forces created during the machining operation. It is desirable to stabilize these unsupported portions by suitable means. When these un-supported portions are or irregular contour their locating positions are not readily predetermined and, therefore, a fixed height support cannot be used. Each successive workpiece may require a different height of supporting device. Another requirement is that, regardless of the device used to support the workpiece, it must not exert an excessive amount of initial contacting force on the workpiece. If the supporting device bears against the workpiece with excessive force, the workpiece will distort from its normal configuration and the machining operation will be performed under this abnormal condition. Thereafter, when the finished workpiece is removed from the work-holding fixture, it will return to its normal shape and the machined portion will not conform to the desired configuration and size.
A variety of devices have been used in the past to solve the above-mentioned problems, but none has been completely satisfactory for various reasons.
Many devices are of course known for such a purpose, but all of these have certain disadvantages. Thus screw-operated jacks are known but these have the disadvantage that excessive load can accidentally be applied to the work-piece when positioning the jack, which may shift or even distort it. Wedge-type jacks are also known which are locked by hydraulic pressure but in the case the load which can be supported is relatively low, and a further notable disadvantage lies in the fact that the load on the jack is transmitted back into the hydraulic system. Again, hydraulically locked work supports are known in which hydraulic pressure is employed to clamp collets or a sleeve about a sliding plunger which supports the workpiece, but the load which can be supported is again limited and also the friction surfaces on the plunger and the sleeve or collets have to be formed to very close tolerances. Other hydraulically locked work supports are known in which hydraulic pressure is employed to lock a series of plates together which provide support to a vertical support plunger. In such devices a spring generated force between the two sets of interleaved plates, one stationary and one movable, generate a transmitted force between the plates which is greater than desired.
Accordingly a principal desirable object of the present invention is to provide an improved device for supporting workpieces.
Another desirable object of the invention is to provide an improved device for supporting workpieces having a movable work supporting plunger which is supported by two relatively movable sets of interleaved friction plates which are clamped together preferably by a hydraulic piston operating at right angles to the plates, to hold the plunger in a desired work supporting position of adjustment.
Other desirable objects and advantages of the invention will in part appear hereinafter and will in part become apparent after consideration of the specification with reference to the accompanying drawing.